The present invention relates to an optical pickup apparatus for recording an information signal onto an optical disk and reading out an information signal recorded on the optical disk from the optical disk by using an optical beam radiated by a single light source at a single wavelength, as well as relates to an optical-disk reproducing apparatus and an optical-disk recording & reproducing apparatus, which employ the optical pickup apparatus, wherein the optical disk can have a plurality of types having different recording densities of the information signal due to different track pitches of recording tracks created on the optical disk.
Conventionally, efforts have been made to increase the recording density of an information signal recorded on an optical disk used as a recording medium for recording the information signal. In the case of a magneto-optical disk having a diameter of about 64 mm, for example, the track pitch of recording tracks for recording an information signal on an optical disk has been reduced from 1.6 microns to 0.95 microns only to be decreased again to 0.55 microns. As a result, there has been proposed a magneto-optical disk having a recording density increased by 5 to 10 or more times.
To record an information signal onto an magneto-optical disk having a track pitch reduced as described above and to read out an information signal recorded on the magneto-optical disk from the magneto-optical disk, it is necessary to decrease the spot diameter of an optical beam used for scanning recording tracks created on the magneto-optical disk.
More specifically, if the spot diameter of the optical beam is large in comparison with the track pitch, tracking cannot be carried out with a high degree of accuracy. That is to say, an information signal cannot be recorded onto or read out from a desired recording track. In addition, the amount of cross talk caused by interference between recording tracks increases, inevitably lowering the SN (Signal to Noise) ratio.
Thus, to reduce the spot diameter of an optical beam radiated to the magneto-optical disk, there has been proposed use of an optical pickup apparatus having a light source for radiating an optical beam with a small wavelength. To record and read out an information signal onto and from a magneto-optical disk having a track pitch of 0.95 or 0.55 microns, for example, there has been proposed an optical-disk recording & reproducing apparatus employing an optical pickup apparatus provided with a semiconductor laser for emitting an optical beam with a wavelength of about 650 nm.
If an optical pickup apparatus provided with a semiconductor laser for emitting an optical beam with such a short wavelength is used to record and read out an information signal onto and from an magneto-optical disk having a track pitch of 1.6 microns, the spot diameter of the optical beam is too small in comparison with the width of the recording track. Thus, tracking of a desired recording track cannot be carried out with a high degree of accuracy. That is to say, an information signal cannot be recorded onto or read out from a desired recording track with a high degree of accuracy.
To enable an optical-disk recording & reproducing apparatus for recording and reproducing an information signal onto and from optical disks such as magneto-optical disks with different recording densities due to different track pitches to serve as an apparatus common to all the optical disks, there has been proposed an optical pickup apparatus having 2 light sources for generating an optical beam with a large wavelength and an optical beam with a small wavelength respectively.
This optical pickup apparatus employed in the optical-disk recording & reproducing apparatus is used by being switched from one of the light sources to the other so as to generate an optical beam with a wavelength appropriate for the track pitch of the optical disk mounted on the recording & reproducing apparatus.
As described above, the optical pickup apparatus having a plurality of light sources requires components for switching the generated optical beam from one optical path to another. As a result, the size of the optical pickup apparatus inevitably increases, making it difficult to employ the unit in an optical-disk recording & reproducing apparatus that is made as small in size as possible.
In addition, the power consumption of a semiconductor laser for emitting an optical beam having a wavelength of 650 nm is greater than the power consumption of a semiconductor laser for emitting an optical beam having a large wavelength, for example a wavelength of 780 nm. An optical pickup apparatus employing a semiconductor laser having a large power consumption is not suitable for a portable optical-disk recording & reproducing apparatus employing a battery as a power source.
Moreover, since a semiconductor laser having a large power consumption also has a large temperature coefficient and a large amount of heat dissipated by the semiconductor itself, it is necessary to take a measure for coping with the thermal dissipation of the semiconductor laser mounted on an optical pickup apparatus to get stable oscillation of an optical beam generated by the optical pickup apparatus. It is thus difficult to implement a small and thin optical pickup apparatus if such a measure is to be taken.
Furthermore, a semiconductor laser for emitting an optical beam having a small wavelength is more expensive than the widely used conventional semiconductor laser for emitting an optical beam having a wavelength of 780 nm. Thus, the cost of the optical pickup apparatus and, hence, the cost of the optical-disk recording & reproducing apparatus employing the optical pickup apparatus cannot be reduced.